Why a Multimeter Cannot Measure the Polarity of Alternating Current
Alternating current (AC) is an electric current whose magnitude and direction change periodically. It has no polarity, only frequency. The frequency of AC power supply in China is 50 hertz, meaning the current cycles back and forth 50 times per second and reverses direction 100 times. This question itself is flawed.
Alternating current has polarity that alternates at a very high speed. Neither analog nor digital multimeters can respond to its instantaneous polarity. The number of polarity changes per unit time is defined as frequency, whose unit is hertz - a physical unit named in honor of physicist Hertz.
Due to the alternating polarity of this type of power supply, ordinary multimeters cannot measure its instantaneous polarity at all. If it is necessary to observe the instantaneous polarity at a certain moment, an electronic instrument called an oscilloscope, dedicated to observing AC or DC waveforms, is required.
The direction of alternating current changes over time, so AC has no fixed polarity. To distinguish live wire and neutral wire, set the multimeter to the high AC voltage range. Hold the black probe by hand and touch the wire with the red probe; the wire causing pointer deflection is the live wire, and the one with no deflection is the neutral wire. In addition, some digital multimeters are equipped with non-contact voltage detection functions for common measurement tasks.
